Inkjet recording systems can form images easily and inexpensively, and thus are used in various fields of printing. One of inkjet recording systems is an active light beam-curable inkjet system that forms an image by allowing droplets of a UV-curable ink to land on a recording medium, and curing the droplets by irradiating with active light beam. The active light beam-curable inkjet system has been gaining attention in recent years since it can form an image having high tight adhesiveness even in a recording medium having no ink absorbability.
In an active light beam-curable inkjet ink, a technology of incorporating a gelling agent in an active light beam-curable inkjet ink for the purpose of preventing coalescence of adjacent dots and further enhancing the scratch resistance of an obtained image, is known. In this technology, the extension of the dots can be prevented by the gelation of the ink due to the crystallization of the gelling agent during the landing. Furthermore, since the viscosity of the ink can be increased even at a low temperature, a printed product that is also excellent in scratch resistance at room temperature can be obtained. For example, in Patent Literature 1, the coalescence of dots is prevented by adding an oil gelling agent to an inkjet ink containing a pigment and a photopolymerizable compound to thereby complete the solidification of the ink droplets that have landed on the recording medium within a short time. In Patent Literature 2, an image having fine toughness is efficiently formed by adding a gelling agent that is gelled at a low temperature to a radiation curable ink containing a pigment or a dye.
Meanwhile, in the case when a pigment is used as a color material for an ink, the dispersibility of the pigment can be increased by incorporating a polymer dispersing agent in the ink, whereby the ink can be stably stored. For example, in Patent Literature 3, a pigment is stabilized by dispersion by adding a polymer dispersing agent in a larger amount than that of the pigment by a weight ratio. Furthermore, in Patent Literature 4, the dispersion stability is further increased by adding a polymer dispersing agent to the ink in a larger amount than that of a pigment by weight ratio, and by further adding a dispersion synergist. At this time, by subjecting the pigment particles to a surface treatment to thereby impart an acidic adsorption functional group such as sulfonic acid, the basic functional group of the polymer dispersing agent and the pigment are made easy to associate, whereby the storage stability of the ink can be enhanced.
As magenta pigments for inkjet inks, quinacridones are known, and among these, when a solid-solution of Pigment Violet 19 (PV19) and Pigment Red 202 (PR202) (hereinafter referred to as PV19/PR202) is used, the color region of the formed image can further be enlarged. Meanwhile, in order to increase the storage stability of an ink containing PV19/PR202, it is important not only to improve the affinity of the polymer dispersing agent to the pigment in the ink, but also to improve the affinity of a photopolymerizable compound to the pigment. On the other hand, by subjecting the pigment particles to a surface treatment to thereby impart alumina to the quinacridone backbone, the affinity of the pigment to the photopolymerizable compound can be enhanced, and thus the storage stability of the ink can be enhanced. The alumina can be imparted to the surfaces of the pigment particles by, for example, the method described in Patent Literature 5 or 6.